Condensate accumulated in an engine intake may get ingested in the engine while the engine is combusting, thereby causing misfires. For example, moisture from humid air may condense on engine components such as the intake manifold and a charge air cooler and form a water puddle locally. Condensate accumulated in the intake manifold or in the charge air cooler (CAC) coupled to the intake manifold may enter the engine cylinders during engine operation, and cause combustion instability.
Various approaches are provided for reducing occurrence of engine misfires due to ingestion of condensate. In one example approach, as shown in US 20160169170, Russ et al. disclose a method to increase airflow through a charge air cooler (CAC) in order to purge condensate from the CAC. A number of engine cylinders of a variable displacement engine (VDE) are selectively deactivated in order to transiently increase air flow via the CAC. During cylinder deactivation, fueling may be disabled to the one or more deactivated engine cylinders while the intake and exhaust valves remain operational, thereby increasing airflow through the CAC. By adjusting the number of cylinders that are deactivated based on an airflow increase required to purge the condensate stored in the CAC, condensate ingestion is reduced.
However, the inventors herein have recognized potential disadvantages with the above approach. As one example, the approach may not be able to address condensation accumulated at one or more engine components during conditions when an engine is not running. The inventors have recognized that during selected conditions, such as during hybrid vehicle propulsion using motor torque from a system battery, during an engine deceleration fuel shut-off condition, or during an engine idle-stop conditions, moisture may accumulate in an engine. In particular, during such engine non-combusting conditions, the engine intake air temperature may fall below a dew point temperature. If the vehicle is travelling through a region having elevated ambient humidity when the intake air temperature falls below the dew point temperature, humid air may enter the intake manifold, such as past a substantially closed intake throttle, and cylinder valves, even if the engine is not running. Since cylinder valves may remain open during a non-combusting condition based on the position of the associated cam lobes at the time when the engine was stopped, the humid air entering the engine may condense in the intake manifold. The moisture may even enter and condense within the cylinders. When the engine is restarted and fuel is subsequently combusted in the engine cylinders, the condensate collected in the intake manifold may be ingested in the cylinders, causing engine misfires and combustion instability.
The inventors herein have recognized that the valve mechanisms of selectively deactivatable cylinders can be controlled so as to hold them in a closed position when desired. By holding them actively closed during conditions when the engine is not combusting and the vehicle is travelling through conditions where condensation in the intake manifold from ambient air is likely, ingestion of moisture in at least those deactivatable cylinders can be reduced. Thus in one example, the issues described above may be addressed by an engine method comprising: responsive to a higher than threshold ambient humidity during an engine non-combusting condition, holding deactivatable cylinder valves closed, and during an immediately subsequent engine combusting condition, activating the deactivatable cylinder valves, and starting combustion in deactivatable cylinders before starting combustion in non-deactivatable cylinders. In this way, when ambient humidity is high while a vehicle is operated with the engine not combusting fuel, valves coupled to deactivatable cylinders may be intentionally held closed to reduce moisture ingestion into the cylinders.
As one example, during engine non-combusting conditions such as vehicle propulsion using motor torque, a deceleration fuel shut-off condition, and an engine idle-stop condition, the engine intake manifold temperature may drop to below a threshold temperature, such as a dew point temperature. If the vehicle is concurrently travelling through regions with higher than threshold ambient humidity, as estimated based on inputs from engine system sensors and/or from an external network communicatively coupled to the vehicle, air saturated with moisture may enter and condense in the intake manifold. To reduce ingestion of this moisture from the intake manifold into the engine cylinders, when the engine is not-combusting fuel, the engine may be rotated using motor torque to engage the valve actuation mechanism of the engine's selectively deactivatable cylinders. The amount of motor torque applied is adjusted so that the valves may be held closed thereby sealing the respective cylinders. Since the engine exhaust manifold may continue to retain heat during engine non-combusting conditions, in response to a higher than threshold exhaust temperature, unburnt fuel and hot exhaust gas may also be routed from the exhaust manifold to the intake manifold via an exhaust gas recirculation (EGR) passage to further reduce intake manifold condensation. The valves may be held closed while the engine is not combusting until an engine restart condition is met. During an immediately subsequent engine combustion event of the restart, the deactivatable cylinders may be reactivated by actuating the corresponding valve mechanisms and combustion may be resumed in the deactivatable cylinders first, while the non-deactivatable cylinders are maintained in the non-combusting condition. After a threshold number of engine cycles have elapsed after resuming combustion in the deactivatable cylinders, combustion may be resumed in the non-deactivatable cylinders while maintaining combustion in the deactivatable cylinders.
In this way, by selectively closing the valves of deactivatable engine cylinders during an engine non-combusting condition, responsive to a higher than threshold ambient humidity, moisture ingestion into the deactivated cylinders and in-cylinder moisture accumulation may be reduced. As a result, misfire occurrence in those cylinders during a subsequent engine operation is also reduced. By recirculating hot residual exhaust through the intake manifold concurrently, moisture condensation in the intake manifold may also be reduced. The technical effect of reactivating the deactivatable cylinders and resuming combustion initially in the deactivatable cylinders (before resuming combustion in non-deactivatable cylinders) during an immediately subsequent engine restart is that the intake manifold temperature may be increased the accumulated moisture may be evaporated. By allowing at least some of the accumulated moisture to be evaporated before initiating combustion in the non-deactivatable cylinders (whose valves cannot be actively held closed), the ingestion of condensate into the non-deactivatable cylinders during cylinder combustion is reduced, thereby increasing combustion stability and misfire occurrence in the non-deactivatable cylinders. By recirculating hot exhaust residuals to the intake manifold during the higher than threshold ambient humidity condition while the engine is not combusting, residual exhaust heat may be effectively used to increase the engine intake temperature, thereby reducing the possibility of intake manifold water puddle formation. Overall, by reducing water accumulation in the intake manifold and within engine cylinders during engine non-combusting conditions, combustion stability may be increased and propensity of misfire may be decreased during an immediately subsequent engine combusting condition.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.